The present invention relates to rotor-shaft assemblies of the type used in exhaust gas driven turbochargers, and more particularly to the attachment of ceramic rotor to a metal shaft assembly.
One means of improving the response time of a turbocharger is to reduce the moment of inertia of the rotating parts by constructing the parts of lighter material, yet the material chosen must be able to withstand the harsh operating environment of the turbocharger. Since the compressor impeller does not see high temperatures in comparison to the turbine wheel, designers began to construct the compressor impellers of low weight aluminum alloy which can survive in the turbocharger environment.
In order to further reduce the weight and therefore the moment of inertia of the rotor-shaft assembly, the industry focused on ceramics as a substitute to the relatively heavy steel turbine wheel. Ceramic substitutes are able to survive the high temperatures and gaseous environment of the turbine. Once the decision has been made to use a ceramic turbine wheel, the focus of attention became the joint between the metal shaft and the ceramic turbine wheel as evidenced by U.S. Pat. Nos. 4,063,850; 4,125,344; and 4,424,003 and German Pat. No. 2,734,797. However, none of these efforts have resulted in a reliable joint as evidenced by the fact that there is no commercially available or production model ceramic turbine wheel on the market, whether it be in turbochargers or any other high speed rotating equipment. Several of these structures teach to shrink fit the ceramic stub shaft of the turbine wheel within a metallic sleeve while others have concentrated on the use of adhesive in order to bond the two materials together.
Utilization of the shrink fit method of attachment gives rise to a further problem: the need to reduce the imposition of the high tensile stresses upon the ceramic stub shaft by the sudden discontinuity of contact between the sleeve member and ceramic rotor. The problem leads to the design feature of scheduling the compressive forces exerted by the sleeve onto the ceramic rotor by substantially tapering the thickness of the sleeve. This reduction in the thickness of the sleeve results in a reduction in the compressive stresses acting on the rotor and the tensile stresses imposed on the ceramic rotor at the point where the contact between the sleeve and rotor ends. It has been found that the tensile and shear stresses which cause the propagation cracks in the ceramic rotor can eventually lead to joint failure.
Furthermore, the high temperature, thermal cycling atmosphere of the turbocharger leads to the degradation and failure of the ceramic rotor-metal shaft joint. Failures occur because of several reasons; the metal sleeve radially expands by a greater degree than the ceramic rotor due to the differential between the two material's coefficient of thermal expansion, thereby loosening the joint (thermal cycling causes "ratcheting", the easing out of the ceramic stub shaft from the sleeve during each cycle) and in the case of adhesives, the breakdown of the adhesive in the high temperature environment.
According to the present invention, a ceramic rotor is attached to a metal shaft via a metal sleeve to form a rotorshaft assembly. The rotor-shaft assembly includes a metal sleeve member having a generally coaxial bore formed therethrough. One end of the sleeve extends generally radially outward to form a hub portion which defines an annular surface area generally coaxial to the shaft. The sleeve hub portion includes an annular groove which is sized to mate with a piston ring located within the center housing near the turbine end of the turbocharger. The ceramic rotor includes a hub and plurality of blades spaced about the circumference of the hub. The rotor further includes a stub shaft integral with and generally symmetrical about the axis of the hub. The stub shaft includes an annular relief therearound. The stub shaft is fitted within the end of the sleeve which defines the sleeve hub portion and the metal shaft is inserted into the other end of the sleeve. Between the ceramic stub shaft and the metal shaft is placed a predetermined amount of braze material. The assembly is heated, thereby melting the braze material which flows into any space between the sleeve and the ceramic stub shaft and metal shaft. Upon cooling, the braze material solidifies and joins the rotor to the shaft.
It is an object of the present invention to provide a ceramic to metal joint for use within a turbocharger.
It is another object of this invention to provide a means from preventing lubricant from entering the turbine housing in the event of a joint failure or ceramic rotor failure.
It is another object of this invention to provide a method of attaching a ceramic shaft to a metal sleeve employing a fluxless brazing operation.
It is a further object to provide a low cost method of joining a ceramic rotor to a metal shaft.